The world is currently witnessing explosive growth in the demand for communications networks and systems and it is predicted that this demand will increase in the future. In particular, there is increased customer demand for IP-based applications, connectivity and services. In addition, there is much focus on the efficiency and scalability of IP-based optical networking infrastructures. As the demand for data services grows at an ever-increasing rate, meeting these demands requires communication networks having higher bandwidth capabilities and higher levels of reliability. In an effort to meet this demand, carriers must install facilities that are capable of carrying increasing amounts of data traffic and which are more resilient in the face of facility failures. Manufacturers of network equipment attempt to keep up with the demand by developing more reliable equipment that can handle higher bandwidths.
In addition, much of the new bandwidth capacity being installed nowadays by carriers includes optical fiber networks. In particular, optical networks based on the SONET and SDH standards are commonly being installed. SONET/SDH optical networks are often configured to operate as ring structures since these types of networks exhibit improved performance.
A diagram illustrating a prior art example of a dual optical ring network including four nodes is shown in FIG. 1. The ring network, generally referenced 10, comprises a plurality of network nodes 12 wherein each node is connected to its two neighbors via optical links. In the example shown herein, the ring network comprises four nodes labeled R1, R2, R3 and R4. Each node is connected to its neighbor via dual optical links to form a bidirectional ring comprising two symmetric counter-rotating fiber rings 14, 16, each of which can be concurrently utilized to pass information and control packets. To distinguish between the two rings, one is referred to as the ‘inner’ ring 16 and the other as the ‘outer’ ring 14. In this case, each ring handles traffic in a single direction, i.e. clockwise traffic and counterclockwise traffic.
The creation of large ring networks containing ten or more nodes is difficult since connections must be made to neighbors on either side of a node. Once a ring network is established and configured, insertion and deletion of nodes typically becomes very problematic. To insert or delete a node, the ring must be broken and new connections established to maintain the integrity of the ring.
One solution to this problem is to use a concentrator that is in essence a network switching device whereby all the network devices (i.e. nodes) are connected in a physical star configuration to the concentrator. The concentrator emulates a ring structure internally by forwarding data from node to node around the ring via internal connections through the switch.
A diagram illustrating a prior art example of a plurality of network nodes connected to a concentrator configured to simulate the operation of a dual optical ring is shown in FIG. 2. The network, generally referenced 20, comprises a plurality of network nodes 22, labeled R1, R2, R3 and R4 wherein each node is connected to a single concentrator 28 via pairs of optical links. The concentrator is configured internally to simulate a ring architecture while the nodes are connected to it in a star configuration. Internal connections 30, 32 are made so as to create a bi-directional ring comprising an inner ring 26 and an outer ring 24.
An example of such a concentrator commercially available is the Cisco ONS15190 IP Transport Concentrator, manufactured by Cisco Systems Inc., San Jose, Calif., which enables the creation of logical rings over physical star-based fiber topology and permits reordering, insertion or removal of nodes on live rings without requiring a service interruption.
Although, connecting all the network nodes (e.g., routers) to a single concentrator simplifies the physical cabling and connectivity needed to establish a ring network, it creates a point of vulnerability in that the failure of a concentrator causes the entire ring to collapse. In the event of a concentrator failure, the nodes have no alternative link available thus preventing communications. On the other hand, in the event a node fails, the remaining nodes still can communicate.
There is thus a need for a mechanism that permits a plurality of network nodes to continue operating as an optical ring in the event of the failure of the concentrator to which each is connected. Such a mechanism would be able to detect the failure of a concentrator and be able to permit the continued operation of the simulated optical ring. The mechanism should provide alternative links between routers in the event a concentrator fails thereby permitting the continued operation of the ring whereby all the nodes participating in the ring can receive and transmit data. Such a mechanism preferably provides alternative links between nodes regardless of the number of nodes located on the ring.